Variable frequency impedance measurement

ABSTRACT

When a defibrillator selects a dosage of energy or current to be delivered to a patient, the defibrillator selects an excitation current frequency and applies the excitation current at the selected frequency to the patient. The frequency of the excitation current is selected as a function of the dosage to be delivered. The patient&#39;s response to the excitation current at the selected frequency will accurately reflect the impedance that the defibrillator will “see” when delivering the selected dosage of energy or current.

CROSS REFERENCE TO RELATED PATENT APPLICATION

This patent application is a division of U.S. Non-Provisional patentapplication Ser. No. 10/176,705, filed on Jun. 20, 2002, now U.S. Pat.No. 8,417,327, which is incorporated by reference.

TECHNICAL FIELD

The invention relates to medical devices, and more particularly, todefibrillators that deliver energy to a patient.

BACKGROUND

A defibrillator is a device that stores energy, typically in one or morehigh-voltage capacitors, and delivers the stored energy to a patient. Inparticular, a defibrillator delivers energy to a heart that isundergoing fibrillation and has lost its ability to contract.Ventricular fibrillation is particularly life threatening becauseactivity within the ventricles of the heart is so uncoordinated thatvirtually no pumping of blood takes place. An electrical pulse deliveredto a fibrillating heart may depolarize the heart and cause it toreestablish a normal sinus rhythm. For some patients, more than onedefibrillation pulse is required.

An external defibrillator applies a defibrillation pulse via electrodesplaced upon the patient's chest. When a switch is closed, current flowsbetween the electrodes and the defibrillator delivers at least some ofthe stored energy to the patient's chest. The dosage of energy deliveredmay be on the order of two hundred joules or more, but the dosagedepends upon the circumstances. The quantity of energy delivered whenthe patient is a child, for example, is generally less than when thepatient is an adult. In some cases, a patient may need multiple shocks,and different dosages may be delivered with each shock.

The energy delivered to the patient, and the current that flows betweenthe electrodes, are a function of the voltage between the electrodes,the impedance of the body of the patient and the pulse width of theshock. For a given voltage and pulse width, a patient having a body witha lower impedance will experience a different current flow, and willthus receive a different quantity of energy, than a patient having abody with a higher impedance. A defibrillator that measures thepatient's impedance accurately can therefore more effectively develop avoltage across the electrodes that accurately delivers a desired dosageof energy or current to the patient. A defibrillator that measures thepatient's impedance accurately can also adjust the shape of the pulsefor enhanced effect.

A defibrillator may measure the impedance of the patient's body byapplying an excitation current, or “carrier,” to the patient via theelectrodes placed upon the patient's chest, and measuring the responseto the application of the excitation current via the electrodes. Theresponse is typically measured as a function of the voltage differencebetween the electrodes.

The excitation current is an alternating current signal, having a smallcurrent magnitude, such as 100 microamperes, and a known frequency. Bymeasuring the magnitude and phase of the response, the patient'simpedance can be determined. Because the patient's body is not purelyresistive but includes a reactive component, the measured impedancevaries depending upon the frequency of the excitation current.Defibrillators that measure impedance with an excitation currenttypically employ one or two particular fixed frequencies, such as 20kHz, 30 kHz or 62 kHz.

The excitation current does not deliver the defibrillation shock. Atypical defibrillation shock is of a substantially higher amperage thanthe excitation current. During delivery of defibrillation shocks, theimpedance exhibited by the patient and “seen” by the defibrillatorvaries. Impedance varies from patient to patient, and may also varywithin a single patient as a function of factors such as the magnitudeof the defibrillation shock. The impedance exhibited by the patientduring delivery of a defibrillation shock may not be the same as theimpedance exhibited during measurement of the response to the excitationcurrent.

SUMMARY

The invention provides techniques for accurately estimating the amountof impedance that a patient will exhibit when a defibrillator delivers adosage of energy or current to the patient. When the defibrillatorselects a dosage to be delivered to the patient, the defibrillatormeasures the impedance of the patient by observing the response toapplication of an excitation current. The defibrillator selects anexcitation current frequency that will accurately reflect the impedancethat the patient will exhibit when the defibrillator delivers theselected dosage.

In one embodiment, the invention is directed to a method in which afrequency of an excitation current is selected as a function of a dosageof energy or current to be delivered to a patient with a defibrillator.The method includes applying the excitation current to the patient atthe selected frequency. The impedance of the patient may be measured bymeasuring a response of the patient to application of the excitationcurrent.

In another embodiment, the invention is directed to a defibrillatorcomprising a processor that selects a dosage of energy or current to bedelivered to a patient and a current source that generates an excitationcurrent at a selectable frequency, the frequency of the excitationcurrent being a function of the selected dosage. The defibrillator mayalso include electrodes that deliver the excitation current to thepatient and an impedance measuring circuit that measures the voltageacross the electrodes and measures the impedance as a function of thevoltage and the excitation current.

In a further embodiment, the invention presents a method comprisingapplying an excitation current of at least two different frequencies toa patient. The method also includes measuring the impedances of thepatient by measuring the responses of the patient to application of theexcitation current at the different frequencies. The method furtherincludes selecting the second frequency as a function of the firstmeasured impedance and as a function of a dosage of energy or current tobe delivered to the patient.

In an additional embodiment, the invention presents a defibrillatorcomprising a current source that generates an excitation current at aselectable frequency, electrodes that deliver the excitation current tothe patient at a selected frequency, an impedance measuring circuit thatmeasures the voltage across the electrodes and measures an impedance asa function of the voltage and the excitation current and a processorthat selects the selected frequency of the excitation current as afunction of a dosage of energy or current to be delivered to thepatient. The processor may also select the dosage.

The invention can provide one or more advantages, including techniquesfor more accurate and effective delivery of a dosage of energy orcurrent. By selecting an excitation current frequency as a function ofthe dosage to be delivered, the defibrillator may obtain data that willallow the defibrillator to charge the energy storage device to a voltagelevel that will accurately deliver the selected dosage. In this manner,the defibrillator is capable of more accurate dosage delivery andtherapy and more accurate determination of patient impedance.Furthermore, the invention may complement and cooperate with otherimpedance-related measurements, such as measurement of body motion ordetection of an improperly applied electrode.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an external defibrillator and apatient.

FIG. 2 is a block diagram illustrating an exemplary embodiment of aprogrammable impedance system.

FIG. 3 is a flow diagram illustrating an exemplary operation of adefibrillator.

FIG. 4 is a chart illustrating an exemplary technique for selection offrequency of an excitation current as a function of an energy dosage tobe delivered to a patient.

FIG. 5 is a flow diagram illustrating another technique for selection offrequency of an excitation current as a function of a dosage to bedelivered to a patient.

DETAILED DESCRIPTION

FIG. 1 is a block diagram showing a patient 10 coupled to an externaldefibrillator 12. Defibrillator 12 administers defibrillation therapy topatient 10 via electrodes 14 and 16, which may be hand-held electrodepaddles or adhesive electrode pads placed on the skin of patient 10. Thebody of patient 10 provides an electrical path between electrodes 14 and16.

Electrodes 14 and 16 are coupled to defibrillator 12 via conductors 18and 20 and interface 22. In a typical application, interface 22 includesa receptacle, and connectors 18, 20 plug into the receptacle. Electricalimpulses or signals may be sensed by defibrillator 12 via electrodes 14and 16 and interface 22. Electrical impulses or signals may also bedelivered from defibrillator 12 to patient 10 via electrodes 14 and 16and interface 22.

Interface 22 includes a switch (not shown in FIG. 1) that, whenactivated, couples an energy storage device 24 to electrodes 14 and 16.Energy storage device 24 stores the energy for a dosage of energy orcurrent to be delivered to patient 10. The switch may be of conventionaldesign and may be formed, for example, of electrically operated relays.Alternatively, the switch may comprise an arrangement of solid-statedevices such as silicon-controlled rectifiers or insulated gate bipolartransistors.

Energy storage device 24 includes components, such one or morecapacitors, that store the energy to be delivered to patient 10 viaelectrodes 14 and 16. Before a defibrillation pulse may be delivered topatient 10, energy storage device 24 must be charged. A microprocessor26 directs a charging circuit 28 to charge energy storage device 24 to ahigh voltage level. Charging circuit 28 comprises, for example, aflyback charger that transfers energy from a power source 30 to energystorage device 24. Because the life of patient 10 may depend uponreceiving defibrillation, charging should take place rapidly so that thedefibrillation shock may be delivered with little delay.

When the energy stored in energy storage device 24 reaches the desiredlevel, defibrillator 12 is ready to deliver the defibrillation shock.The shock may be delivered automatically or manually. When the shock isdelivered automatically, microprocessor 26 activates an input/output(I/O) device 32, such as an indicator light or a voice prompt, thatwarns the operator that defibrillator 12 is ready to deliver adefibrillation shock to patient 10. The warning informs the operator ofthe impending shock so that no one other than patient 10 will receivethe defibrillation shock. Microprocessor 26 then activates the switch toelectrically connect energy storage device 24 to electrodes 14 and 16,and thereby deliver a defibrillation shock to patient 10. In the case ofa manual delivery, microprocessor 26 may activate an I/O device 32 thatinforms the operator that defibrillator 12 is ready to deliver adefibrillation shock to patient 10. The operator may activate the switchby manual operation, such as pressing a button, and thereby deliver adefibrillation shock to patient 10.

Microprocessor 26 may also modulate the electrical pulse delivered topatient 10. Microprocessor 26 may, for example, regulate the shape ofthe waveform of the electrical pulse and the duration of the pulse.

Microprocessor 26 may perform other functions as well, such asmonitoring electrocardiogram (ECG) signals sensed via electrodes 14 and16 and received via interface 22. Microprocessor 26 may determinewhether patient 10 suffers from a condition that requires adefibrillation shock, based upon the ECG signals. In addition,microprocessor 26 may also evaluate the efficacy of an administereddefibrillation shock, determine whether an additional shock iswarranted, and the magnitude of energy to be delivered in the additionalshock.

The goal of defibrillation is to depolarize the heart with electricalcurrent and cause the heart to reestablish a normal sinus rhythm. Insome patients, one shock is insufficient to reestablish normal rhythm,and one or more additional defibrillation shocks may be required. Beforeanother shock may be administered, however, charging circuit 28ordinarily must transfer energy from power source 30 to energy storagedevice 24, thereby recharging energy storage device 24. In rechargingenergy storage device 24, as in the initial charging, time is of theessence, and charging circuit 28 therefore charges energy storage device24 quickly. The energy or current dosage delivered to patient 10 neednot be the same in each shock.

Power source 30 may comprise, for example, batteries and/or an adapterto an exterior power source such as an electrical outlet. In addition tosupplying energy to charging circuit 28 and energy storage device 24,power source 30 also supplies power to components such as microprocessor26 and I/O device 32, e.g., via a power supply circuit (not shown inFIG. 1).

Defibrillator 12 further includes programmable impedance system 34. Aswill be described in more detail in connection with FIG. 2, programmableimpedance system 34 includes a controllable current source forgenerating an “excitation current,” also called a “carrier.” Theexcitation current is applied to patient 10 through interface 22 andelectrodes 14 and 16. Programmable impedance system 34 also includes animpedance measuring circuit that measures the impedance of the body ofpatient 10 via electrodes 14 and 16 and interface 22. The impedancemeasuring circuit detects the response to the excitation current as atime-varying voltage difference between electrodes 14 and 16. Bymeasuring the magnitude and phase of the voltage difference,programmable impedance system 34 measures the impedance of patient 10.

The excitation current may be an alternating current signal of knownmagnitude and frequency. The excitation current is much smaller inmagnitude than the defibrillation current delivered during delivery ofan energy or current dosage. A typical excitation current has amagnitude of around 100 microamperes. When applied to patient 10, theexcitation current is typically constant in magnitude and frequencyduring the period of application. The frequency of the excitationcurrent may be varied, however, and the frequency employed during oneperiod may be different from the frequency employed during anotherperiod. For example, programmable impedance system 34 may, during afirst period, apply an excitation current to patient 10 with a constantfrequency of 30 kHz. During a later period, the programmable impedancesystem 34 may apply an excitation current to patient 10 with a constantfrequency of 60 kHz.

The impedance measured using an excitation current at one frequency willusually not be the same as the impedance measured using an excitationcurrent at a different frequency. A human body demonstrates bothresistive and reactive components, so the measured impedance of patient10 varies depending upon the frequency of the excitation current.

Impedance measurements may be stored in memory 36. Memory 36 may storeother data as well, such as vital signs of patient 10 and the therapydelivered to patient 10. In addition, memory 36 may store instructionsthat direct the operation of microprocessor 26. Memory 36 may includevolatile storage, such as random access memory, and/or non-volatilestorage, such as flash memory or a hard disk.

In one embodiment of the invention, programmable impedance system 34selects a frequency for the excitation current as a function of theenergy or current to be delivered to patient 10 in a pendingdefibrillation shock. The energy or current dosage delivered in adefibrillation shock is a function of the voltage difference acrosselectrodes 14 and 16 and the impedance of patient 10 between electrodes14 and 16. The impedance of patient 10, however, varies as a function ofthe magnitude of the defibrillation pulse.

In defibrillation, the energy delivered to patient 10, rather than thevoltage difference developed across electrodes 14 and 16, is usually thequantity of interest. The energy delivered to patient 10 varies with thecurrent delivered via electrodes 14 and 16, and so defibrillation mayalso be quantified in terms of the delivered current. For purposes ofsimplicity, the invention will be described in terms of a dosage ofenergy, but the invention encompasses dosages of current as well.

To deliver a desired dosage of energy with reasonable accuracy,therefore, the impedance of patient 10 should be measured withreasonable accuracy, and the measured impedance should be close to theimpedance that defibrillator 12 will see when delivering thedefibrillation shock. Defibrillator 12 may then control the voltageacross electrodes 14 and 16 to deliver the desired dosage of energy, andmay also control the waveform of the electrical pulse and the durationof the pulse. The invention provides techniques for selecting afrequency for the excitation current that will accurately reflect theimpedance that patient 10 will exhibit and that defibrillator 12 willsee when delivering the defibrillation shock. With this feature,defibrillator 12 can be controlled to avoid delivering a dosage ofenergy that is too high or too low for desired effectiveness.

FIG. 2 is a block diagram illustrating an exemplary embodiment ofprogrammable impedance system 34. Programmable impedance system 34supplies an excitation current 40 to interface 22 (not shown in FIG. 2).A voltage 42 across electrodes 14 and 16 (not shown in FIG. 2) is aninput to programmable impedance system 34.

Excitation current 40 is supplied by a controlled current source 44.Current source 44 is controlled by a programmable drive 46, whichregulates the frequency of excitation current 40. Controller 48 selectsthe frequency of excitation current 40 and supplies the selectedfrequency to programmable drive 46. Current source 44 may generateexcitation current 40 along a continuous range of frequencies, or maygenerate excitation current 40 at discrete frequencies. In a typicalapplication, current source 44 may generate excitation current 40 atthree or more frequencies, and controller 48 may select the frequency ofexcitation current 40 from at least three frequencies.

In one embodiment of the invention, controller 48 selects the frequencyof excitation current 40 as a function of the dosage of energy to bedelivered to patient 10 in a pending defibrillation shock. The energy tobe delivered is one element of system data 50 that may be supplied toprogrammable impedance system 34.

Programmable impedance system 34 supplies excitation current 40 to thebody of patient 10, and measures the response. Voltage 42 is supplied toan amplifier 52, which finds the voltage difference 54 betweenelectrodes 14 and 16. Amplifier 52 may also perform some filtering ofnoise from the input signal.

Amplifier 52 is the gateway between programmable impedance system 34 andinterface 22. Accordingly, excitation current 40 is channeled throughamplifier 52. In addition, amplifier 52 may provide protection toprogrammable impedance system 34 from electrical surges.

Programmable filter 56 receives voltage signal 54. Programmable filter56 may be a band pass filter with a variable center frequency.Controller 48 selects the center frequency and supplies the centerfrequency to programmable filter 56. Although controller 48 may alsoadjust the bandwidth of programmable filter 56, the bandwidth isordinarily narrow, to reject substantially all frequencies except thefrequency of excitation current 40. When controller 48 supplies theselected frequency to programmable drive 46, controller 48 also suppliesa corresponding center frequency to programmable filter 56.

A detector 58 receives filtered signal 60. Detector 58 recovers thesignal or signals that represent the measure of impedance. The signalsmay include, for example, a magnitude signal and a phase signal, fromwhich the resistive and reactive components of the impedance may befound. Alternatively, the signals may express the impedance as a realpart and an imaginary part. In general, the measured impedance is equalto measured voltage 54 divided by applied excitation current 40, wherevoltage 54 and current 40 are complex.

Detector 58 may be tuned by controller 48 to recover the signals at thefrequency of excitation current 40. Recovered signals 62 may beconverted to digital signals 64 by analog-to-digital (A/D) converter 66for processing or transmission by controller 48. Controller 48 may relaythe measured impedance to other components of defibrillator 12. Inparticular, the impedance measured by programmable impedance system 34may be used by microprocessor 26 to select the amount of energy to bestored in energy storage device 24. In a typical application,microprocessor 26 controls charging circuit 28 to increase capacitorcharge voltage of energy storage device 24 to a specified level, so thata desired dosage of energy may be delivered to patient 10.

FIG. 2 shows an exemplary logical relationship among the components ofprogrammable impedance system 34, but is not limited to any particularhardware or software implementation. For example, some components, suchas programmable filter 56 and detector 58, may be realized as analogcomponents, digital components, or a combination of analog and digitalcomponents. A/D converter 66 may be located so as to convert analogsignals to digital signals where needed.

Furthermore, programmable impedance system 34 may include additionalcomponents that are not shown in FIG. 2, such as a bandpass filter toshape and remove noise from excitation current 40. Programmableimpedance system 34 may also exclude components that shown in FIG. 2.The functions of controller 48, for example, may be performed bymicroprocessor 26. The invention encompasses all of these variations.

FIG. 3 is a flow diagram illustrating an exemplary operation ofdefibrillator 12. Upon placement of electrodes 14 and 16 on the chest ofpatient 10, microprocessor 26 analyzes ECG signals from the heart ofpatient 10 (70). Microprocessor 26 determines whether patient 10 suffersfrom a condition that requires a defibrillation shock, based upon theECG signals (72). If defibrillation is not indicated, no shock isdelivered, but monitoring of the ECG may continue (74).

If defibrillation is indicated, the defibrillation sequence starts. Inparticular, microprocessor 26 controls charging circuit 28 to begincharging energy storage device 24 (76). Microprocessor 26 also selects adosage of energy to be delivered to patient 10 (80). For the firstdefibrillation shock, the dosage delivered may be a default dosage ofenergy, such as 200 joules. The dosage selected may also depend uponinitial conditions supplied to defibrillator 12 by an operator (78),such as the sex, approximate age or approximate weight of patient 10.

Delivery of the selected dosage of energy to patient 10 depends upon thevoltage developed across electrodes 14 and 16, which is a function ofthe capacitor charge voltage of energy storage device 24. Delivery ofthe selected dosage of energy to patient 10 also depends upon theimpedance of the body of patient 10. Accordingly, programmable impedancesystem 34 proceeds to measure the impedance of patient 10.Microprocessor 26 or programmable impedance system 34 selects afrequency for excitation current 40 (82) and programmable impedancesystem 34 delivers excitation current 40 to patient 10 via electrodes 14and 16 (84).

The frequency selected for excitation current 40 is a function of theselected dosage of energy to be delivered to patient 10. Techniques forselection of the frequency of excitation current 40 will be describedbelow. When programmable impedance system 34 applies excitation current40 to patient 10 at the selected frequency and measures the voltage thatresults from application of excitation current 40 (86), programmableimpedance system 34 obtains a reasonably accurate measurement of theimpedance that patient 10 will exhibit and that defibrillator 12 willsee when delivering the energy in a defibrillation shock (88).

Microprocessor 26 charges energy storage device 24 to a voltage thatwill deliver the selected dosage of energy, taking into considerationthe measured impedance of patient 10 (90). When energy storage device 24has stored sufficient energy, defibrillator 12 delivers a defibrillationshock (92). The electrical pulse may be modulated to improve thedelivery of energy depending on the measured impedance of patient 10. Asnoted above, modulation may include regulation of the shape of thewaveform of the electrical pulse and the duration of the pulse.

In addition to measuring impedance prior to delivery of thedefibrillation shock, defibrillator 12 may measure impedance duringdelivery of the defibrillation shock as well (94). This measurement mayhelp gauge the accuracy of the impedance measurement made by observingthe response to application of excitation current (84, 86, 88).

After delivery of a defibrillation shock, the ECG of the patient ismeasured again (70) to evaluate the efficacy of the shock (72). In somecases, the heart of patient 10 fails to respond to a defibrillationshock, and another shock, often delivering an increased dosage ofenergy, is indicated. Programmable impedance system 34 measures theimpedance of patient 10 again by selecting an excitation currentfrequency as a function of the new dosage (82), delivering theexcitation current (82) and measuring the voltage across electrodes 14and 16 (86).

FIG. 4 is a chart illustrating an exemplary technique for selection offrequency of excitation current 40 as a function of the dosage of energyto be delivered to patient 10. The horizontal scale 100 shows the energyto be delivered, in joules. The horizontal scale 100 is logarithmic. Thevertical scale 102 represents the frequency of excitation current 40, inkilohertz.

The data points in the chart have been derived by experimentation. Inparticular, the data points represent the excitation current frequenciesthat were used to measure patient impedance for different dosages ofenergy, with the least amount of error. The equation of the “best fit”line 104 of the data points is 9.7876*ln(x)+13.56, with an R-squaredvalue of 0.7891. The data points may also be approximated with a curveother than a straight line. In general, for a lower energydefibrillation shock, a lower excitation current frequency results in amore accurate impedance measurement, and consequently a more accuratedelivery of energy to patient 10.

Upon selecting a dosage of energy for delivery, microprocessor 26 orprogrammable impedance system 34 may select an excitation currentfrequency by applying the equation illustrated in FIG. 4. Alternatively,microprocessor 26 or programmable impedance system 34 may select anexcitation current frequency by consulting a lookup table that mapsdefibrillation energy to excitation frequency.

The excitation current frequency may also be a function of factors inaddition to defibrillation energy, such as the sex, age or body mass ofpatient 10, the impedance measurement obtained during delivery of aprevious defibrillation shock, the number of defibrillation shockspreviously delivered, or the shape and duration of the waveform of theelectrical pulse. Accordingly, a chart, such as the chart shown in FIG.4, need not include a single curve, but may include a family of curves.Similarly, there may be a family of equations or lookup tables thatrelate an energy dosage and an excitation current frequency.

FIG. 5 illustrates a technique in which programmable impedance system 34may select an excitation current frequency as a function of at least oneother measurement of impedance. A first excitation current frequency isselected (110). The first excitation current frequency may be a standardor default frequency, and need not be a function of the dosage to bedelivered to patient 10. A first excitation current is delivered at thefirst frequency (112). The voltage that results from application of theexcitation current at the first frequency is measured (122), and a firstimpedance is computed (124). Based upon the first measured impedance, anexcitation current function is selected. An excitation current functionmay be, for example, a curve, equation or lookup table that relatesexcitation current frequency to an energy or current dosage.Microprocessor 26 or programmable impedance system 34 may, for example,select one curve when the measured first impedance is high and adifferent curve when the measured first impedance is low.

Microprocessor 26 or programmable impedance system 34 selects afrequency for the excitation current according to the dosage and theexcitation current function (120). A second excitation current at theselected frequency is delivered (122), and a second impedance ismeasured (124, 126). The second measured impedance may represent anestimate of the impedance that defibrillator 12 is likely to see whendelivering the dosage. Microprocessor 26 may regulate the charging ofenergy storage device 24 to a voltage that will deliver the desireddosage, taking into consideration the measured impedance of patient 10(128). Defibrillator 12 may thereafter deliver a defibrillation shockhaving the desired dosage (130). In this way, one or more additionalimpedance measurements may be used to obtain a more accurate selectionof the excitation current frequency that will measure the impedancedefibrillator 12 will see when delivering a dosage to patient 10.

The additional impedance measurements may be used for other purposes aswell. For example, the difference between the first and second impedancemeasurements may be indicative of the body mass of patient 10. Thedifference between the first and second impedance measurements may alsoindicate whether defibrillator 12 is coupled to a patient. At times,defibrillator 12 may be coupled to a test device to determine whetherdefibrillator 12 is operating properly. A typical test device includes aresistive element, such as a high-power fifty ohm resistor, to simulatethe patient. The simulation is somewhat inaccurate, however, because theimpedance of a patient's body typically includes both resistive andreactive components, while the test device has a negligible reactivecomponent. By measuring a first and second impedance, therefore,defibrillator 12 may be able to recognize that the delivery of a dosageof energy or current is a test rather than an actual defibrillation.When defibrillator 12 detects that a test is underway, defibrillator 12may automatically enter a testing mode. When in testing mode,defibrillator 12 may, for example, prevent data becoming stored inmemory 36, thereby reducing the risk of accidentally overwriting actualpatient data with test data, or may identify the stored data as testdata.

Single or multiple impedance measurements, e.g., by continuous sampling,may be useful in many other contexts as well. For example, themeasurement of impedance is also affected by the motion of patient 10.When the patient is not moving voluntarily or being moved by others(e.g., by administration of cardiopulmonary resuscitation), the changesof impedance at a fixed excitation current frequency may be indicativeof involuntary motion, such as respiration. In other words, whenelectrodes 14 and 16 are first applied to patient 10, defibrillator 12may measure both the ECG and the respiration rate of patient 10.

Impedance measurements may also indicate whether an electrode isproperly affixed to the skin of patient 10. If an electrode comes loose,then the measured impedance will increase dramatically. An I/O device 32may advise the operator that an electrode has come loose. The above usesof impedance measurements are complementary and are non-exclusive. Theinvention may operate in harmony with impedance measurements made forany other purpose.

The invention can provide one or more advantages. In particular, theinvention provides techniques for more accurate, and therefore moreeffective, delivery of energy to a patient. By selecting an excitationcurrent frequency as a function of the energy to be delivered, thedefibrillator may obtain a more accurate estimate of the impedance thepatient may exhibit when the defibrillator delivers the defibrillationshock. As a result, the energy storage device may be charged moreclosely to a proper voltage level, and the shape and duration of thedefibrillation pulse may be adapted to the impedance of the patient. Anadditional benefit is that the range of tolerance for the componentssuch as the energy storage device is reduced, thereby savingconstruction costs.

As mentioned above, the invention is not limited to dosages quantifiedaccording to units of energy. Dosages may also be quantified accordingto units of current delivered in a defibrillation shock. The techniquesdescribed above may also be employed to select an excitation currentfrequency as a function of the defibrillation current to be delivered.

Various embodiments of the invention have been described. Theseembodiments are illustrative of the practice of the invention. Variousmodifications may be made without departing from the scope of theclaims. For example, programmable impedance system 34 may measureimpedance by any of a number of impedance-measuring techniques. Theinvention is not limited to measuring impedance by measuring a voltage.Nor is the invention limited to an excitation current of a singlefrequency, but encompasses embodiments in which an excitation currentincludes multiple frequency components. These and other embodiments arewithin the scope of the following claims.

What is claimed is:
 1. A method comprising: applying a first excitationcurrent at a first frequency to a patient; measuring a first impedanceof the patient by measuring a response of the patient to application ofthe first excitation current; selecting a second frequency of a secondexcitation current as a function of both the first measured impedanceand a dosage of at least one of energy and defibrillation current to bedelivered to the patient with a defibrillator.
 2. The method of claim 1,further comprising: applying the second excitation current to thepatient at the selected second frequency; and measuring a secondimpedance of the patient by measuring a response of the patient toapplication of the second excitation current.
 3. The method of claim 2,further comprising selecting an excitation current function as afunction of the first measured impedance.
 4. The method of claim 2,wherein the excitation current function comprises an equation thatrelates excitation current frequency to an energy or current dosage. 5.The method of claim 2, wherein the excitation current function comprisesa lookup table that relates excitation current frequency to an energy orcurrent dosage.
 6. The method of claim 2, further comprising finding thedifference between the first impedance and the second impedance.
 7. Themethod of claim 1, wherein the defibrillator is an externaldefibrillator.
 8. A defibrillator comprising: a current sourceconfigured to generate an excitation current at a selectable frequency;at least two electrodes configured to deliver the excitation current toa patient at a selected frequency; an impedance measuring circuitconfigured to measure the voltage across the electrodes and to measurean impedance as a function of the voltage and the excitation current;and a processor configured to select the selected frequency of theexcitation current as a function of a dosage of at least one of energyand defibrillation current to be delivered to the patient.
 9. Thedefibrillator of claim 8, wherein the current source is configured togenerate a first excitation current at a first frequency and a secondexcitation current at a second frequency, and wherein the impedancemeasuring circuit is configured to measure a first impedance associatedwith the first excitation current at the first frequency and a secondimpedance associated with the second excitation current at the secondfrequency.
 10. The defibrillator of claim 9, wherein the processor isconfigured to select the second selected frequency as a function of thefirst impedance.
 11. The defibrillator of claim 8, wherein the processoris configured to select a dosage of at least one of energy anddefibrillation current to be delivered to the patient.
 12. Thedefibrillator of claim 8, wherein the defibrillator is an externaldefibrillator.